You must be wondering why the hell would I want to do that and why not just develop in 1.4? Well, for one, may be you want to use the new features of JDK1.5 but some of the customer for some reason still stuck with JDK1.4 or like in my case, due to some stupidity on our client's part that he bought license to an older version of the application server (yes, it was Websphere) which does not support 1.5 and rest of the stupidity on our part that we just assumed that the application server will be supporting 1.5.
We kept on developing the application and giving demos to the client on our server and when it came to deployment only then we realized that we are in one big soup. And as always Google to the rescue, which helped me find an open source solution Retroweaver which operates by transforming Java class files compiled by a 1.5 compiler into version 1.4 class files which can then be run on any 1.4 virtual machine.
For more details follow the link. Hope this helps 
